class Instrument:
    def __init__(self, name):
        self.name = name

    def make_sound(self):
        pass


class Erhu(Instrument):
    def make_sound(self):
        print('Erhu is playing')


class Piano(Instrument):
    def make_sound(self):
        print('Piano is playing')


class Violin(Instrument):
    def make_sound(self):
        print('Violin is playing')


def play(inst):
    inst.make_sound()


erhu = Erhu('Erhu')
piano = Piano('Piano')
violin = Violin('Violin')
erhu.make_sound()
piano.make_sound()
violin.make_sound()
